Flash memory device having a graded composition, high dielectric constant gate insulator

ABSTRACT

A graded composition, high dielectric constant gate insulator is formed between a substrate and floating gate in a flash memory cell transistor. The gate insulator comprises amorphous germanium or a graded composition of germanium carbide and silicon carbide. If the composition of the gate insulator is closer to silicon carbide near the substrate, the electron barrier for hot electron injection will be lower. If the gate insulator is closer to the silicon carbide near the floating gate, the tunnel barrier can be lower at the floating gate.

RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.11/811,609 titled “FLASH MEMORY DEVICE HAVING A GRADED COMPOSITION, HIGHDIELECTRIC CONSTANT GATE INSULATOR”, filed Jun. 11, 2007 now U.S. Pat.No. 7,892,921 that is a Divisional of Ser. No. 11/114,403, now U.S. Pat.No. 7,253,469 titled “FLASH MEMORY DEVICE HAVING A GRADED COMPOSITION,HIGH DIELECTRIC CONSTANT GATE INSULATOR” filed Apr. 26, 2005 which iscommonly assigned and incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to memory devices and inparticular the present invention relates to flash memory devices withgraded composition gate insulators.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuits in computers or other electronic devices. There aremany different types of memory including random-access memory (RAM),read only memory (ROM), dynamic random access memory (DRAM), synchronousdynamic random access memory (SDRAM), and flash memory.

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Common uses for flash memory include personal computers, personaldigital assistants (PDAs), digital cameras, and cellular telephones.Program code and system data such as a basic input/output system (BIOS)are typically stored in flash memory devices for use in personalcomputer systems.

Flash memory cells are typically comprised of field effect transistors(FET) with floating gates. The gates are referred to as floating sincethey are electrically isolated from other conductive areas of thetransistor by layers of oxide insulation. The floating gate can beprogrammed or erased by Fowler-Nordheim tunneling in which electronstunnel through a barrier in the presence of a high electric field in theoxide.

One drawback with floating gate FETs is the relatively large amount oftime needed to store a charge on the floating gate during a writeoperation and the relatively large amount of time necessary to removethe charge during an erase operation. One reason for the high timerequirements is the relatively large tunneling barrier between thesilicon substrate and the silicon dioxide insulator. Additionally, thehigh electric field required to cause electron injection in order totunnel through the barrier typically contributes to reliability problemsand premature gate insulator breakdowns.

As a typical prior art example, silicon dioxide (SiO₂) is an insulatorwith a relative dielectric constant of 3.9, an energy gap ofapproximately E_(g)=9 eV, and electron affinity of χ=0.9 eV. Bycomparison, the energy gap and electron affinity for the semiconductorsilicon are E_(g)=1.1 eV and χ=4.1 eV, respectively. In a conventionalflash memory cell, electrons stored on the polysilicon floating gate seea large tunneling barrier of about 3.2 eV. FIG. 1 illustrates thetypical prior art large barrier, Φ=3.2 eV, for tunneling erase in flashmemory devices. The large tunneling barrier Φ=3.2 eV is the differencebetween the electron affinities of silicon (i.e., χ=4.1 eV) and SiO₂(i.e., χ=0.9 eV). This is a relatively large barrier that requires ahigh applied electric field.

There is a resulting need in the art for an improved gate insulator thatprovides a low tunneling barrier in order to decrease the time requiredfor programming and erase operations in a flash memory cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical prior art electron energy diagram for a flashmemory cell.

FIG. 2 shows a cross-sectional view of one embodiment of a flash memorycell transistor of the present invention having a graded composition,high dielectric constant gate insulator.

FIG. 3 shows one embodiment of an electron energy band diagram inaccordance with the graded composition, high dielectric constant gateinsulator transistor structure of FIG. 2.

FIG. 4 shows an electron energy band diagram in accordance with anotherembodiment of the graded composition, high dielectric constant gateinsulator transistor structure of FIG. 2.

FIG. 5 shows a plot of band gap energies versus electron affinities inaccordance with the embodiments of the present invention.

FIG. 6 shows a band diagram for amorphous silicon carbide on silicon inaccordance with the embodiments of the present invention.

FIG. 7 shows a band diagram for germanium carbide on germanium inaccordance with the embodiments of the present invention.

FIG. 8 shows a block diagram of an electronic system of the presentinvention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings that form a part hereof and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. In the drawings, like numerals describesubstantially similar components throughout the several views. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilizedand structural, logical, and electrical changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims and equivalents thereof. The terms wafer or substrate used in thefollowing description include any base semiconductor structure. Both areto be understood as including silicon-on-sapphire (SOS) technology,silicon-on-insulator (SOI) technology, thin film transistor (TFT)technology, doped and undoped semiconductors, epitaxial layers of asilicon supported by a base semiconductor structure, as well as othersemiconductor structures well known to one skilled in the art.Furthermore, when reference is made to a wafer or substrate in thefollowing description, previous process steps may have been utilized toform regions/junctions in the base semiconductor structure, and termswafer or substrate include the underlying layers containing suchregions/junctions.

FIG. 2 illustrates a cross-sectional view of one embodiment of a flashmemory cell transistor of the present invention. The transistor has agraded composition, high dielectric constant (i.e., high-K) gatedielectric. The transistor comprises a pair of source/drain regions 201and 202 implanted in a silicon substrate 211. The direction of operationof the transistor determines which region 201 or 202 functions as asource and which functions as a drain. A channel region 203 existsbetween the source/drain regions 201 and 202. In an alternateembodiment, the substrate 211 comprises germanium.

In one embodiment, the source/drain regions 201 and 202 are n+ dopedregions in a p+ type substrate 211. An alternate embodiment may use p+doped source/drain regions in an n+ type substrate. The presentinvention is not limited to any one conductivity type for thesource/drain regions or the substrate.

A high-permittivity (high-K), graded composition tunnel gate dielectric206 is formed over the substrate 211 substantially between thesource/drain regions 201 and 202 and over the channel region 203. Asdiscussed subsequently, the tunnel gate dielectric 206 is amorphousgermanium carbide or graded composition amorphous germanium-siliconcarbide that provide adjustable barrier heights in contact with thesubstrate 211 and floating gate layer 208.

A floating gate layer 208 is formed over the tunnel gate dielectriclayer 206. A gate insulator layer 210 is formed over the floating gate208. A control gate 212 is formed on top of the gate insulator layer210. In one embodiment, the floating gate layer 208 and the control gate212 are a polysilicon material and the gate insulator layer 210 is aninterpoly insulator.

The architecture of the flash memory cell transistor of FIG. 2 is forpurposes of illustration only. The amorphous germanium carbide andgraded composition amorphous germanium-silicon carbide tunnel dielectricof the present invention is not limited to planar transistors asillustrated in FIG. 2. This type of tunnel dielectric can be used inother types of planar transistors as well as vertical transistors.

In one embodiment, a high dielectric constant is considered to be adielectric constant that is greater than that of SiO₂. A wide variety ofdifferent high dielectric constant insulators can be realized usingatomic layer deposition (ALD) or evaporation techniques. An example ofsuch dielectric materials and their characteristics are summarized inthe following table:

Dielectric Band Gap Delta E_(c) (eV) Material Constant (K) E_(c) (eV) toSi SiO₂ 3.9 8.9 3.2 Si₃N₄ 7 5.1 2 Al₂O₃ 9 8.7 2.8 Y₂O₃ 15 5.6 2.3 La₂O₃30 4.3 2.3 Ta₂O₅ 26 4.5 1-1.5 TiO₂ 80 3.5 1.2 HfO₂ 25 5.7 1.5 ZrO₂ 257.8 1.4 SiC 10 3.4 1.1 Ge_(X)C_(Y) 12 (est.) 5.7 2.2

Alternate embodiments use other dielectrics than those listed above thathave other dielectric constants. The characteristics of these materialsare well known to those skilled in the art and are not discussedfurther.

The high-K dielectric materials of the present invention can be used asgraded composition in either the tunnel gate dielectric 206 or the gateinsulator layer 210. By varying the composition ratios of these high-Kdielectrics, the barriers and/or tunnel barriers can either be reducedbetween the silicon and the gate dielectric as illustrated in FIG. 3 orbetween the floating gate and the gate insulating dielectric asillustrated in FIG. 4.

FIG. 3 illustrates an electron energy band diagram in accordance withthe graded composition, high dielectric constant gate insulatortransistor structure of FIG. 2. The graded composition, in oneembodiment, is silicon carbide to germanium carbide that is graded inthe direction shown from silicon carbide closest to the siliconsubstrate to germanium carbide closest to the polysilicon floating gate.By making the insulator composition close to silicon carbide at thesubstrate, the electron barrier for hot electron injection will bereduced from the prior art. Also, the tunnel barrier from the floatinggate can be kept higher by making the insulator composition close togermanium carbide at the floating gate.

This composition could be deposited on a silicon carbide gate insulatorformed by carburization of silicon. Alternate embodiments could useother techniques for the deposition of silicon carbide and germaniumcarbide.

An alternate embodiment uses a graded composition of silicon oxide tosilicon carbide to germanium carbide. As illustrated in FIG. 3, thetunnel gate insulator is graded from silicon oxide nearest the substrateto silicon carbide to germanium carbide nearest the floating gate.

In still another embodiment, the gate insulator comprises amorphousgermanium carbide. As seen in the table above and the plot of FIG. 5,this composition also provides a lower band gap and larger electronaffinity than prior art materials such as SiO₂.

FIG. 4 illustrates an electron energy band diagram in accordance withanother embodiment of the graded composition, high dielectric constantgate insulator transistor structure of FIG. 2. The graded composition,in one embodiment, is silicon carbide to germanium carbide. The gateinsulator is graded in the direction shown from silicon carbide nearestthe polysilicon floating gate to germanium carbide nearest the siliconsubstrate. FIG. 4 shows that this graded composition results in a lowbarrier at the polysilicon floating gate-gate insulator interface.

In one embodiment, the germanium carbide is deposited on the siliconsubstrate or a thin layer of silicon oxide and the composition gradedtowards silicon carbide as the tunnel gate insulator is furtherdeposited. Alternate embodiments can use other techniques for formingthis layer.

In another embodiment, the tunnel gate insulator comprises a gradedcomposition of silicon oxide to silicon carbide to germanium carbide. Asindicated in the figure, the silicon oxide is formed closest to thefloating gate while the germanium carbide is formed closest to thesilicon substrate.

FIG. 5 illustrates a plot of electron affinity versus band gap energy ofsilicon, silicon carbide (SiC), Germanium, and Germanium Carbide(Ge_(X)C_(Y)). As shown in this plot, prior art insulator silicondioxide has a relative dielectric constant of 3.9, energy gap ofapproximately 9.0 eV, and electron affinity of 0.9 eV. In a conventionalflash memory, electrons stored on the polysilicon floating gate see alarge tunneling barrier of about 3.2 eV. This value is the differencebetween the electron affinities of silicon (4.1 eV) and SiO₂ (0.9 eV).This is a relatively large barrier that requires high applied electricfields for electron injection.

The plot also shows that SiC has a band gap of 3 eV and an electronaffinity of 3.7 eV. Amorphous SiC has relatively low conductivity undermodest applied electric fields. Similarly, Ge_(X)C_(Y) has an estimatedband gap of 2.5 eV and an electron affinity of 3.0 eV. Both have a muchlarger electron affinity and a much smaller barrier than SiO₂ thatrequires a much lower applied electric field for electron injection.

FIG. 6 illustrates a band diagram for amorphous silicon carbide onsilicon in accordance with the embodiments of the present invention. Ifamorphous hydrogenated silicon carbide is deposited with a gap of up to3.4 eV, amorphous silicon carbide can then have a band gap of greaterthan 2.1 eV of crystalline SiC and an electron affinity of less than 3.7eV. Amorphous silicon carbide on silicon has a low surface recombinationvelocity and excellent passivation on silicon as compared to the priorart silicon dioxide.

FIG. 7 illustrates a band diagram for germanium carbide on germanium inaccordance with the embodiments of the present invention.Microcrystalline hydrogenated germanium carbide films have beendeposited by RF sputtering and electron cyclotron resonance plasmaprocessing. With a low carbon concentration of 4%, these have a band gapenergy of around that of silicon (i.e., 1.2 eV). Amorphous hydrogenatedgermanium carbide can be deposited as an insulator and it can have,after annealing, a band gap energy as high as 7.1 eV and an electronaffinity of 1.2 eV. Amorphous germanium carbide on germanium can have anenergy barrier of up to 2.8 eV and is therefore suitable for passivationand a dielectric insulator.

FIG. 8 illustrates a functional block diagram of a memory device 800that can incorporate the flash memory cells of the present invention.The memory device 800 is coupled to a processor 810. The processor 810may be a microprocessor or some other type of controlling circuitry. Thememory device 800 and the processor 810 form part of an electronicsystem 820. The memory device 800 has been simplified to focus onfeatures of the memory that are helpful in understanding the presentinvention.

The memory device includes an array of flash memory cells 830. Thememory array 830 is arranged in banks of rows and columns. The controlgates of each row of memory cells is coupled with a wordline while thedrain and source connections of the memory cells are coupled tobitlines. As is well known in the art, the connection of the cells tothe bitlines depends on whether the array is a NAND architecture, a NORarchitecture, or some other array architecture. The flash memory cellsof the present invention are not limited to any one architecture.

An address buffer circuit 840 is provided to latch address signalsprovided on address input connections A0-Ax 842. Address signals arereceived and decoded by a row decoder 844 and a column decoder 846 toaccess the memory array 830. It will be appreciated by those skilled inthe art, with the benefit of the present description, that the number ofaddress input connections depends on the density and architecture of thememory array 830. That is, the number of addresses increases with bothincreased memory cell counts and increased bank and block counts.

The memory device 800 reads data in the memory array 830 by sensingvoltage or current changes in the memory array columns usingsense/buffer circuitry 850. The sense/buffer circuitry, in oneembodiment, is coupled to read and latch a row of data from the memoryarray 830. Data input and output buffer circuitry 860 is included forbi-directional data communication over a plurality of data connections862 with the controller 810). Write circuitry 855 is provided to writedata to the memory array.

Control circuitry 870 decodes signals provided on control connections872 from the processor 810. These signals are used to control theoperations on the memory array 830, including data read, data write, anderase operations. The control circuitry 870 may be a state machine, asequencer, or some other type of controller.

The flash memory device illustrated in FIG. 8 has been simplified tofacilitate a basic understanding of the features of the memory. A moredetailed understanding of internal circuitry and functions of flashmemories are known to those skilled in the art.

CONCLUSION

In summary, flash memory transistors of the present invention withgraded composition, high-K gate dielectrics reduce the electron barrierbetween the substrate and gate insulator and the tunnel barrier betweenthe polysilicon floating gate and gate insulator. This is accomplishedby using a graded composition, high-K dielectric gate insulator insteadof the prior art's silicon dioxide.

The use of the gate insulators described above allows for two differentbarriers. The lower height of the tunneling barriers with high-Kdielectric gate insulators can provide larger tunneling currents out ofthe floating gate with smaller control gate voltages. The lower barrierat the interface with the silicon substrate makes the write operationeasier and write currents by channel hot electron injection larger atlower voltages. Both tunneling currents and hot electron injectioncurrents are exponential functions of the barrier heights and electricfields.

The higher dielectric constant gate insulators of the present inventionalso allow for better scaling of flash memory devices to smallerdimensions. The effective gate length of high-K gate tunnelingdielectric flash memory transistors can be scaled below 50 nm. High-kgate dielectrics reduce or eliminate drain turn-on problems,short-channel effects and punchthrough in flash memory transistors.Smaller write and erase voltages provide another advantage in that thethickness of the SiO₂ or other insulator layer between the control gateand the floating gate can be reduced.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

1. A memory cell, comprising: a silicon substrate; a charge storagestructure comprising a polysilicon floating gate; a control gate; atunnel dielectric comprising a graded composition, the gradedcomposition is a silicon carbide to a germanium carbide that is gradedin a direction from the silicon carbide closest to the silicon substrateto the germanium carbide closest to the polysilicon floating gate, andan insulator between the charge storage structure and the control gate.2. The memory cell of claim 1, wherein the graded composition furthercomprises a graded composition of silicon oxide to silicon carbide togermanium carbide between the semiconductor and the charge storagestructure.
 3. The memory cell of claim 1, wherein the charge storagestructure comprises a floating gate.
 4. The memory cell of claim 3,wherein the floating gate comprises a polysilicon floating gate.
 5. Thememory cell of claim 1, wherein the control gate comprises a polysiliconcontrol gate.
 6. The memory cell of claim 1, wherein the memory cellcomprises a planar memory cell transistor.
 7. The memory cell of claim1, wherein the memory cell comprises a vertical memory cell transistor.8. The memory cell of claim 1, wherein the semiconductor is one ofsilicon or germanium.
 9. The memory cell of claim 1, wherein the gradedcomposition of silicon carbide to germanium carbide comprises a greateramount of silicon carbide at an interface between the semiconductor andthe tunnel dielectric.
 10. The memory cell of claim 1, wherein thegraded composition of silicon carbide to germanium carbide comprises agreater amount of silicon carbide at an interface between the chargestorage structure and the tunnel dielectric.
 11. The memory cell ofclaim 1, wherein the memory cell is part of a NAND non-volatile memoryarray.
 12. The memory cell of claim 1, wherein the tunnel dielectric hasa dielectric constant greater than 3.9.
 13. The memory cell of claim 1,wherein the tunnel dielectric has an energy gap of approximately 9.0 eVand electron affinity of 0.9 eV.